why is armor being countered by items it is supposed to counter

Confusing title?

let me clear that up

All of the armorpen items give either aheavy amount of AD, or a at least a good amount of AD in addition to some extra benefits

Armorpen in and of itself is a powerfull stat, it allows you to counteract someone's defenses in order to deal relevant damage.

Right now though armorpen is tacked onto some of the most potent AD items barring crit items, thank god for that one.

And this is an issue

Because you can opt into grabbing penetration to deal more damage to an armored target without sacrificing anything for that decision in fact it rewards you for doing by giving you improved capability to fight everything including non armored/defense building targets

Let me give an example

Lets start with This item gives 50 AD thats quite a decent amount people might argue but hey its not 75 like bloodthirster But that is just the thing its stronger and not because of the penetration, +15% physical damage vs targets with higher health Let's break that apart, these days almost anyone who doesn't actually ebenfit from this item does have more then 500 health over anyone who does. 15% is quite alot let me show you how. Let's say you get 300 AD total, which isn't too hard to get, On an auto attack this means you gain the equivalent of 45 AD in bonus damage. Abilities are all different in scaling but they also have base damage which also gets increased so overal lets stick to 45 AD for simplicity sake 50 Base AD and the passive easily being worth 45AD makes this item the highest flat damage item available period! Lets break it down a bit further with 15% bonus damage it would give more flat damage then the highest flat AD item at a mere 200 total ad!

And then you still get the bonus 45% armorpen on top of it !!

Ok that breakdown took a bit longer then i expected but i hope it gets the point across

TL;DR Armorpenetration items are overloaded. They hardly have trade off's if they have any at all! Meanhwile they counter their counter itmezitation pretty effectively leaving no buildpath available to defend or workaround it.